Digital display spectrum analyzers are well known in the art. One example of a digital display spectrum analyzer is the model 71000A modular spectrum analyzer manufactured by Hewlett-Packard Company ("HP"), Palo Alto, California. Analog display spectrum analyzers are also well known in the art. The model 8558 and 8559 spectrum analyzers manufactured by HP are exemplary.
Analog and digital display spectrum analyzers each have their own advantages and disadvantages, but digital display spectrum analyzers have gained greater popularity as a result of their greater bandwidth and flexibility as well as their display stability and consistency. Nonetheless, analog display spectrum analyzers have several desirable features which heretofore have not been realized in digital display spectrum analyzers due to inherent limitations of the latter.
For example, analog display spectrum analyzers exhibit a characteristic known as "overwriting" wherein the CRT beam can retrace part or all of a prior trace or traces to provide highly varying intensities along the X-Y axes. Both the relative intensity level of any portion of a trace, as well as the intensity variation along a trace, can be significant and may convey important information, for example, relative amount of Z-axis modulation of the displayed signal. Known digital display spectrum analyzers are generally incapable of displaying a waveform in this manner.
The problem is best exemplified by reference to FIGS. 1 and 2. FIG. 1 illustrates traces of a signal waveform plotted on a conventional analog display spectrum analyzer such as the HP model 8558 or 8559. (As used herein, the term "signal waveform" is not limited to a time vs. amplitude characterization of the signal, but as used in connection with spectrum analyzers also includes a frequency vs. amplitude characterization of the signal.) A peak 100, as well as subsidiary peaks 110 which may represent, for example, noise peaks or harmonics, are clearly visible. Noise 120 under the peaks is also clearly visible. Of particular interest, however, is the varying intensity of the peaks 100, 110 which would be clearly visible on an actual display of an analog display spectrum analyzer. (Actual intensity variations cannot be effectively illustrated herein due to limitations in reproducing an actual trace with solely black ink on a white medium.) Noise 120 along the X-Y axes is also of interest. As mentioned, the relative intensity level, as well as the intensity variations of both the peaks 100, 110 and the noise 120 may convey important information.
FIG. 2 illustrates a trace of the same signal waveform plotted on a conventional digital display spectrum analyzer such as the HP 71000A. As can be seen, not only are the subsidiary peaks 110' and noise not readily ascertainable, but in an actual display the intensity level of the trace would also be relatively constant. Thus, the information normally conveyed by relative intensity levels across the X-Y axes in an analog display spectrum analyzer is lost.
FIGS. 1 and 2 also illustrate another important advantage of analog display spectrum analyzers and corresponding disadvantage of digital display spectrum analyzers. Referring to FIG. 1, an underpeak 140, as well as "ripples" 160 along the base line of the signal waveform are clearly visible in the analog display. However, referring to FIG. 2, the corresponding underpeak 140', as well as the corresponding "ripples" 160' are not readily ascertainable from the digital display, and indeed might easily be overlooked without first having had the benefit of seeing the display of FIG. 1.
Still further, analog display spectrum analyzers provide trace "persistence" since trace data on the CRT is not immediately lost from trace to trace. Thus, multiple traces may be simultaneously plotted and displayed even though the input signal may have been digitally processed by the spectrum analyzer prior to display. Known conventional digital display analyzers generally do not plot multiple traces of a given input signal, and hence do not provide trace persistence.
The following examples illustrate the importance of the information that is "lost", or otherwise not readily ascertainable, in a conventional digital display spectrum analyzer. In a standard NTSC television signal, there is occasionally interference in the frequency area between the video carrier and the color carrier. If this interference is not frequency stable, or is weak, it may be difficult to detect on a conventional digital display spectrum analyzer due to the lack of intensity variation and persistence. Similarly, so-called "black-burst" television signals may exist when a first weak signal exists below a second stronger signal in the frequency spectrum. The first (weaker) signal exhibits an "underpeak" 140 readily ascertainable in an analog display spectrum analyzer (FIG. 1); however the underpeak is not readily ascertainable in a conventional digital display spectrum analyzer (FIG. 2). The so-called "Hannover Blind" effect often seen on a television screen (as the result of displaying a striped or checkered pattern) can be indicated by "ripples" readily ascertainable in an analog display spectrum analyzer. For example, "ripples" 160 (albeit not due to the Hannover Blind effect) are clearly ascertainable in FIG. 1, but not in FIG. 2.
Pulsed signals, such as those found in radar or FM applications, are not clearly ascertainable on a conventional digital display spectrum analyzer when one of the signals is stronger than another, due to the inability of a conventional digital display spectrum analyzer to plot the varying intensities of the trace of the signal waveform. Both signals would be readily visible and ascertainable on an analog display spectrum analyzer. Another example is the examination of television gray scales. It is possible to determine the amplitude of gray levels with an analog display spectrum analyzer because it is capable of displaying varying intensities, but not on a conventional digital display spectrum analyzer. Other examples where an analog display spectrum analyzer provides superior visual signal information include gated applications such as wideband local area networks, intermod distortion of television signals, differentiation of carrier feed-through power for radar signals, and situations in which it is necessary to differentiate between a plurality of signals that are superposed.
Notwithstanding the foregoing limitations, digital display spectrum analyzers offer important advantages over analog display spectrum analyzers, and further, digital display spectrum analyzers readily interface with other types of digital test equipment. It is therefore desirable to provide a digital display spectrum analyzer that retains all of the benefits of conventional digital display spectrum analyzers, but also provides the advantages of analog display spectrum analyzers discussed above. The present invention achieves this goal.